Degradation of Dryland Vegetation Patchiness Through the Lens of Power-Law Relationships

Vegetation patchiness is a key characteristic of drylands and closely linked with their functioning. Deviations from pure power-law distributions of patch sizes have been suggested as indicators of degradation and early warning signals of approaching desertification thresholds. The general objective of our study is to evaluate the usefulness of patch-size distributions as indicators of vegetation degradation caused by grazing and drought. To achieve this, we utilized field data and data generated by individual-based simulation models to examine how vegetation patchiness changes in Patagonian drylands under different rainfall scenarios and grazing conditions. First, we compared patch-size distributions predicted by two models for semi-arid grass steppes and for arid shrub steppes under current rainfall conditions with field data from two grazing scenarios (ungrazed and overgrazed). Second, we used the models to simulate the long-term effects of grazing on vegetation patchiness under both drier and wetter conditions. Our field data revealed that the impact of grazing depends on aridity, with stronger effects observed in the semi-arid steppes than in the arid steppes. Our simulation results indicate that changes in rainfall regimes have a stronger effect on patch-size distributions than grazing. Wetter conditions increased the number of medium-to-large-sized patches in both steppes, regardless of grazing. Deviations from power-law distributions were not directly related to overgrazing and degradation; however, grazing and changes in rainfall caused systematic changes in the parameters of the power-law patch-size distributions, supporting their usefulness as one indicator in the assessment of degradation.